PayPal does this too. They will offer to do the currency conversion at an outrageous rate. Not quite 15%, though always substantially more than Mastercard’s rate of the day.

The even worse part about PayPal is that they have a whole system of nonsensical fees to fall back to when you inevitably figure out how to evade the obvious ones. For instance, sidestepping their dynamic currency conversion by temporarily changing which currency they bill on your card (which by the way is rate limited to only a few times per month) will result in another "non-foreign transaction but with recipient in foreign country" fee appearing, covering the inherent costs of converting German US dollars to American US dollars or whatever. They will at least hide the fee from you for business transactions, but the merchant still has to pay it.

Amazon does something like this too, though I'm not sure of the percent. I just know that every time I select to pay in dollars, any change in delivery options will select it back to pay in euros, where the bank is.

I genuinely don't know if this is good or not, but the UIs insistence on reverting back to another currency after my initial selection leads me to believe that my initial selection hits them the hardest the most